The measurement of viscosity of a fluid is important in many industries and applications where fluid properties need to be monitored and controlled.
In biology and medicine, viscosity measurements are important for understanding a variety of complex biological fluids, such as engineered polymers, the interior of living cells, and blood. These measurements aid in the fundamental understanding of biological processes and the ability to monitor and control human disease. For example, blood viscosity is an important parameter in monitoring diabetic patients and patients receiving blood transfusions. Many industrial products including food products, household and industrial chemicals, pharmaceutical products, and cosmetics are complex fluids, and the ability to monitor viscosity therefore has many potential applications in industrial or pharmaceutical R&D, process monitoring, and quality control.
Relaxometry measurements have been used to evaluate cytoplasmic motions, rheology, and structure using magnetic nanoparticle probes. For example, Valberg and Feldman, The Journal Of Cell Biology, volume 101, pp. 130-140, 1985, as well as Valberg and Albertini, Biophysical Journal, Volume 52, pp. 551-561 1987, describe testing of hamster lung cells with ingested ferromagnetic nanoparticles by first subjecting the cells to a brief magnetizing field pulse having an initial orientation followed by measurement of the decay of the remanent magnetic field (relaxation) of the cells in a weaker, re-oriented magnetic field that imparts torque to the nanoparticles. The re-orientation of the magnetic fields allows the driven particle rotation to be evaluated and intracellular apparent viscosity determined from the rate of particle rotation. Spontaneous (Brownian) relaxation of the nanoparticles (i.e. without the weaker, re-oriented) magnetic field is also described.